Electrical transmitting system



Dec. 3; R MORGAN v 2,224,000

' ELECTRICAL TRANSMITTING s sTEM Filed Jan. 30, 1937 is i Awe 1700/0 J/a/v/u 16 Ol/ECE I inventor Hdolph H.Mor gan attorney Patented Dec. 3, 1940 PATENT OFFICE 2,224,000 ELECTRICAL TRANSMITTING SYSTEM Adolph Morgan, Merchantvillc, N. .L, assignor to Radio Corporation of America, a corporation of Delaware Application January 30, 1937, .Serial No. 123,099

2 Claims.

This invention relates to electrical transmittin systems, and more particularly to a system employing a plurality of channels for the reproduction of sound.

In the copending application of Charles M. Burrill, Serial No. 82,374, now Patent No.

2,097,995, issued Nov. 2, 1937, assigned to the Ra-.

dio Corporation of America, there is disclosed a sound reproducing system wherein a plurality of channels are employed for transmitting signal impulses'from a plurality of records on which the same sound is recorded, but at a difierent level on each record. Only those sections of the several records which are least distorted by background noise and by overmodulation are utilized in reproductiomthe proper signal current being selected after transmission and equalization of outputs in the individual channels (for example, amplifiers) respectively associated with each record.

The primary object of my invention is to provide a simpler and faster operating means, than is disclosed in said copending application, for electrically switching from one channel to another.

More specifically, it is an object of my invention to provide an improved transmission system wherein one or more audio channels, each transmitting voltages substantially equal in amplitude and phase, may be switched or connected to another audio channel without the production of an audible click in the latter channel when the switching is effected.

Another object of my invention is to provide an improved transmission system as aforesaid wherein the current supplied to the single audio channel is substantially independent of the number of supplycircuits contributing current thereto, whereby the insertion or withdrawal of one or more of said supply circuits will produce no audible effect in the single channel.

It is also an object of my invention to provide an improved transmission system of the type set forth which is simple in construction and highly efficient in use. 1

In accordance with my invention, I provide a plurality of series connected input channels or circuits, each of which includes a source of voltage anda limiting resistance in series with the voltage source, the voltage source and limiting resistance in each channel being of such values that the current supplied to a receiving channel which is in series with all of the input circuits is substantially independent of the impedance of the receiving circuit, within limits, and of the number of channels efiectively connected in series with it. The receiving circuit includes a resistance in shunt with the input circuits, this resistance being low compared with the resistances of each of the input circuits. Witheach 5 input circuit, I provide switch means for shortcircuiting the same, whereby one or more of the input circuits may be short-circuited at will, and since the current supplied to the receiving circuit is substantially unchanged, no audible click will 10 result.

The novel features that I consider characteristic of my invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and 15 method of operation, together with additional objects and advantages thereof, will best be understood from the following description of two embodiments thereof, when read in connection with the accompanying drawing, in which Figure 1 is a circuit diagram of one form of my invention, and

Figure 2 is a circuit diagram of a somewhat difierent form thereof.

Referring more specifically to the drawing, there is shown, in Figure 1, a pair of series connected input circuits I- and 2 in series with a receiving circuit 3, such as an amplifying channel. The input circuits I and 2 each include a source of alternating voltage 4 such as an audio pre-amplifier fed by a source of audio signals (not shown), and a current limiting resistance 5 in series with the source 4, while the receiving circuit 3 has a resistance 6 connected in shunt with both input circuits I and 2, the shunt resistance 6 having a small value compared with each of the resistances 5. The voltage sources 4 may, for example, comprise the signal sources in thesystems of the above-mentioned copending application of Charles M. Burrill, and would therefore be substantially equal in value. It follows that the resistances 5 would then be sub stantially equal in value. Now, if the value of the resistance 6 is low compared with that of the resistances 5, it follows that thecurrent supplied to the receiving circuit 3 is practically constant regardless of whether only the channel I is feeding the channel 3 whether only the channel 2 is feeding the channel 3, or whether both of the channels I and 2 are simultaneously feeding the receiving channel 3. An arrangement of this sort, therefore, permits switching from one input circuit to the other or to both, or vice versa, without producing an audible disturbance in the receiving circuit 3. since the current supplied to the latter is substantially constant and independent of the number of supply or input circuits supplying current thereto.

For enabling the operator to select, at will, the appropriate supply channel which is to be operative during any particular time interval, a double throw switch I may be connected between the series connected input circuits I- and 2 by means of a lead 8. Connected to a point common to one end of the resistance 6 and the limiting resistance 5 of the channel I is a lead 9, while a lead I is similarly connected to a point common to the other end of the resistance 6 and the limiting resistance of the channel2. With the switch "I in its mid-position, as shown in Figure 1, the channels I and 2 both supply current to the receiving channel 3. When the switch I is thrown into contact with the lead 9, the channel .I becomes short-circuited and the lead 8 is connected through the switch I and the lead 9 to the upper end of the resistance 6. Only the channel 2 then supplies current to the channel 3, but because the voltages of the sources 4 of channels I and 2 are substantially equal, and because of the low value of the resistance 6 compared with that of the resistances 5, the current then supplied to the channel 3 is practically the same as when both input channels were connected thereto, and therefore no audible click results. By throwing the switch I into contact with the lead III, the channel 2 is short-circuited and the lead 8 is connected to the lower end of the resistance 6 through the switch I and the lead I0, whereby only the channel I will supply current to the receiving channel 3. Again, however, the current supplied to the channel 3 is practically the same as before, and therefore no audible click will result. The switch I may be arranged to operate automatically in response, for example, to a suitable control voltage derived at some point in the system, or in response to a separate predetermined control means.

In the modification of the invention shown in Figure 2, four input circuits II, I2, I3 and I4 are connected in series with each other by means of the leads I5, I6 and I1 and inseries with the receivingcircuit 3. In shunt with each of the input circuits. I I, I2, I3 and I4 are the switches I8, I9, 20 and 2|, respectively. Any one or more of the input channels may be short-circuited by closing their respective shunting switches, whereby only that one or those several input circuits whose switch or switches, as the case may be, are open will supply current to the receiving circuit 3. For example, by closing the switches I8, 20 and 2|, the channels II, I3 and M will be short-circuited and current will be supplied to the receiving channel 3 only by the channel I2. Similarly, shouldthe switches I9 and 20 alone be closed, onlythe channels I I and I4 will supply current to the receiving channel 3, the circuit being completed from the channel I I to the channel I4 through the lead I5, the switch I9, the lead I6, the switch 20 and the lead II. In any case, however, since the current supplied to the receiving .channel 3 is practically constant, no

audible click will result. As in the case of the switch I of Figure -1, the switches I8, I9, 20 and 2| .may be arranged to operate automatically according to the input circuit or circuits that are to be temporarily operative to supply current to the receiving circuit 3-.

The operation of this invention depends on the condition that the currents in each of the input circuits be substantially equal both in magnitude and in phase. This condition is met if the voltage of the sources 4 are equal in magnitude and in phase, and if resistances 5 are equal. The voltage sources 4 should be equal in phase but need not be equal in magnitude provided resistances 5 are proportionately unequal. When the voltages from the sources 4 are derived from separate sound records, the equality in phase must be secured'by exceptionally accurate synchronization. Usual methods of synchronization are only concerned with average speeds whereas here instantaneous speeds must be equal. One method of obtaining the accurate synchronization required which has been found entirely practical is to make all the records simultaneously on a single film.

It is desirable that the above condition of equality in magnitude and phase be fully met. However if the differences obtained in practice are reasonably small, the transient disturbance resulting from switching from one input circuit to another, caused by these differences, will not produce seriously disturbing audible clicks in receiving channel 3.

Although I have described but two embodiments of my invention, it will be apparent to those skilled in the art that many other modifications thereof are possible..

For example, resistance 6 in connection with receiving circuit 3 may be omitted, provided receiving circuit 3 without resistance 6 has a suitable resistive impedance low compared with resistances 5. Moreover, while I have disclosed my invention as particularly useful for a system such as described in the above-identified copending application, it should be understood that it is by no means limited thereto. Thus, my invention may find use in a system wherein a plurality of signal sources are synchronized for either successive operation or for simultaneous operation. I therefore desire that my invention shall not be limited except insofar as is made necessary by the prior art and by the spirit of the appendedclaims.

I claim as my invention:

1. In an electrical audio transmission system, the combination of a plurality of serially connected audio input circuits each including a voltage source and a current limiting resistance in series with said voltage source, an audio amplifying circuit in series with said input circuits, said amplifying circuit having a low resistance compared with the resistance of each of said input circuits, and means for selectively short'circuiting one or more of said input circuits.

2. The invention set forth in claim 1 characterized in that the values of the voltage sources and current limiting resistances in each of said input circuits are such that constant current is supplied to said amplifying circuit regardless of the number of said input circuits connected to said amplifying circuit at any time whereby no audible click will result upon either coupling of said input circuits to said amplifying circuit or uncoupling said input circuits therefrom.

ADOLPI-I R. MORGAN. 

